Apply A Style Expression During Runtime?

I need to set the position of an element to fixed using JavaScript at runtime.
Sadly, IE6 does not know this value, so I want to use the following workaround:

Instead of setting the position to fixed, I set it to absolute for IE6, and the top value needs to be the following expression:

Code:

top: expression(0+((e=document.documentElement.scrollTop)?e:document.body.scrollTop)+'px');

Sadly I have no idea on how to apply this expression to myFixedObj.style.top! I tried it using

Code:

myFixedObj.style.top=expression(0+((e=document.documentElement.scrollTop)?e:document.body.scrollTop) +'px');

and

Code:

myFixedObj.style.top="expression(0+((e=document.documentElement.scrollTop)?e:document.body.scrollTop)+'px')";